Added w3id based import statements to chem_dcat_ap schemas

[HendrikBorgelt]

This PR changes the Import statements in the chem_dcat_ap profiles, chem_dcat_ap, chemical_entites_ap, and chemical_reaction_ap (material_entites_ap was already changed via the last release).

This makes reusing the schema in another GitHub project simpler, since you don't need to have the subschemas stored in your local project.

[HendrikBorgelt]

Implementing the imports via w3id's is not as clean as I would have liked it to be. Since our w3id's point to the GitHub pages, one must first update the GitHub pages before one is beeing able to check the correct implementation or the next nested level of the schema imports. Since I did not want to do too many PR and generate their respective Release Candidates, I updated the schemas in the GitHub pages manually. If we merge this PR, the GitHub pages should be fixed and conform to the schemas of this PR.

If you have any remarks on what should be changed, like the request from Philip, feel free to request this, but I can not guarantee that we will get a successful PR merge/RC out of it. Which might make the changes even messier.

[HendrikBorgelt]

@dalito, How do you think we should proceed here?

Should we revert the pull requests and then rather go with regular PR's and RC's like you mentioned in #92 ?
I don't think that something like a force-push from the PR's before my pullrequest or a rebase would be feasible.

Or should we continue merging this PR?

[HendrikBorgelt]

PR #123 should fix this issue anyway, so no need to keep this issue open.